The present invention relates to a thin film-shaped electrode adapted to contact with a cornea or a conjuctiva bulbi for obtaining an electroretinogram as general information or record concerning the action potentials of stratum retinae induced by stimulus of light irradiation.
It has already been adopted as one of the routine examinations to detect electric potential shift of the retina induced by stimulus of light irradiation to learn the action or exitation of the neuroepitheliale retinae and the stratum ganglionare retinae to utilize the means or supplementing data for diagnosis of oculopathy. In such an examination, it is a common practice to place a conductor (differential electrode) in fornix conjunctivae or on the cornea to detect the potential shift between a reference electrode (indifferent electrode) bonded to the forehead or ear lobe, the thus detected shift in potential being amplified and recorded, in order to measure the potential of the retina which is embedded deeply in the orbita without subjecting the eyeball organisms to any invasion. Such a technique has been disclosed for example, in Yoshihito Honda, "Eye and Electrophysiology (Ophthalmologic MOOK (14))", page 284 (1980), Kanehara Shuppan K.K.; Yoshihito Honda, "General Examination Methods in Ophthalmology (Ophthalmologic MOOK (3))", page 137 (1978), Kanehara Shuppan K.K.; and Isamu Tsukahara and Eli Sakaue "Textbook of Ophthalmology", page 156 and page 168 (1968), Kanehara Shuppan K.K. As the indifferential electrode used in this examination, a silver plate electrode analogous to the electrode used in the electroencephalography may be effectively used with the silver chloride face thereof firmly contacting the skin of the ear lobe or the like.
However, a fully satisfactory differential electrode which is fitted directly on the cornea or conjunctiva has not yet been developed, and research continues to develop an electrode suited for such an application to satisfy the requirement that damage of the eyeball organisms should be minimised. (In this connection, reference should be made to Ryoji Asayama, Makoto Nagata et al., "Jap. J. of Clinical Ophthalmology" vol. 11, 304 (1957) and J. G. F. Worst et al., Am. J. Ophthalomol., vol 51, 410 (1961).) The use of silver foil, silver-plated Nylon string, silver-coated hard contact lens, gold-coated hard contact lens and platinum-coated hard contact lens has been proposed, for example, by Kenji Yanashima et al., "Jap. J. of Clinical Ophthalmology" vol. 37, 777 (1983); Heiichiro Kawabata, "Annual Review of Japanese Ophthalmological Society", vol. 18, 848 (1967); and Yasuo Kubota, "Journal of the Japanese Ophthalmological Society", vol. 67, 1985 (1963) and "Jap. Rev. of Clinical Ophthalmology" vol. 76, 430 (1982). It has also been proposed to use cotton impregnated with physiological saline solution, carbon fibers or a hard contact lens coated with carbon fibers, for example, by Hiroya Sato, "Jap. J. of Clinical Ophthalmology" vol. 25, 743 (1971), and Daizo Yonemura et al., "Ophthalmology", vol 13, 455 (1971). However, all of the prior art means has at least one defect or disadvantage such as the uncomfortable feeling caused by a foreign body, difficulty in firm fitting, irritation or damage of the tissue, drying of the cornea or electrical noises due to the photoelectric effect or Becquerel effect.
On the other hand, it is desirous that a "disposable" differential electrode be developed in order to obviate cross-infection between patients subjected to the examination. However, it is not expected to reduce the production cost of any one of the electrodes described above considerably if they are made disposable since they are produced through complicated production processes. Under such circumstances, it is inevitable that it takes time for sterilizing a differential electrode prior to the repeated use thereof.
It has been tried to use a soft contact lens for such a purpose. However, in addition to the cost problem, the mechanical strength of a soft contact lens is reduced as the water content, i.e. the content of physiological saline solution contained therein, of the material for the soft contact lens is increased to meet the requirement that the conductivity thereof be increased. It has been pointed out that even a soft contact lens for common use containing a relatively small amount of water (ranging from 35% to 55%) is seriously contaminated with bacteria or fungi, thus requirring scrupulous and frequent sterilization, and that a soft contact lens is apt to be broken or damaged during handling operations including sterilization, fitting and detachment, as reported by Yutaka Mizutani, "Contact Lens (Ophthalmologic MOOK (2))", page 45 (1978), Kanehara Shuppan K.K., and by Yoichi Ohta, ibid., page 212. A soft contact lens which is improved in mechanical strength to some extent and contains about 70% of water has been proposed recently. However, the soft contact lens according to this recent proposal is detrimental in frequent daily handling operations including frequent repeated fitting ad detaching operations and scrupulous sterilization operation. Another disadvantage of the soft contact lens according to this recent proposal is that it is relatively expensive since it is produced through a complicated production process involving a spin-casting or a lathe-cut step, which poses a serious barrier to use it as a disposable lens.
With the aim of eliminating direct contact of an electrode with the eyeball tissue, it has been proposed to use a skin electrode which is applied to angulus oculi medialis or the lower eyelid. Such proposals are disclosed by Yoshihito Honda, "Ophthalmology", vol. 17, page 135 (1975); and Y. Honda et al., J. Pediatri. Ophthalmol. Strab., 16, 62 (1979) and 20, 153 (1983). Although many disadvantages of the conventional technology can be solved by adopting the last-mentioned proposal, such means is detrimental in S/N ratio (signal to noise ratio) and inferior in reproducibility of the result of measurement since the electric signal from the eyeball tissue is not detected directly.